1. Field of the Invention
This invention relates to an automatic frequency control apparatus for an FSK receiver and FSK receiver including the same.
2. Description of the Prior Art
An automatic frequency control apparatus for an FSK receiver is known. In a direct conversion FSK receiver, if there is a difference between a carrier frequency and a local oscillation frequency, a sensitivity will decrease because one intermediate frequency after quadrature conversion will increase but the other intermediate frequency after the quadrature conversion will decrease. Accordingly, an error rate in the data demodulation on the decreased intermediate frequency side increases. Therefore, in such a direct conversion FSK receiver, a prior art automatic frequency control (AFC) apparatus is used. Such a prior art automatic frequency control (AFC) apparatus is disclosed in Japanese patent application provisional publication No. 4-45636.
FIG. 19 is a block diagram of a prior art automatic frequency control apparatus. FIG. 20 shows waveforms of signals at respective points of the prior art automatic frequency control apparatus.
A received FSK signal derived from the frequency shift keying modulation with a binary digital signal indicative of a mark or space is inputted into an input terminal 101 and is amplified by an amplifier 102. An output of the amplifier 102 is supplied to mixers 103 and 104. A voltage controlled oscillator (VCO) 105 generates and supplies a local oscillation signal to the mixer 103 and to a 90.degree.-degree phase shifter 106. The 90.degree.-degree phase shifter 106 shifts the local oscillation signal by 90.degree. degrees and supplies the phase shifted local oscillation signal to the mixer 104. Lowpass filters 107 and 108 limit passbands of the outputs of the mixers 103 and 104 respectively to provide an in-phase base band signal (I signal) and a quadrature base band signal (Q signal) respectively. A modulation circuit 801 effects demodulation with these I and Q signals to provide a demodulated output 808.
A limiter circuit 802 limits an amplitude of the Q (I) signal as shown by a waveform 152. An edge detection circuit 803 detects an edge in the output of the limiter circuit 802 as shown by a waveform 153. A pulse generation circuit 804 generates a pulse having a constant duration in response to an output of the edge detection circuit 803 as shown by a waveform 154. An integrating circuit 805 integrates the pulses from the pulse generation circuit 804 to provide a frequency to voltage conversion (F/V conversion) of the base band signal as shown by a waveform 155. The result of the F/V conversion is averaged by a mean value circuit 806 to provide a voltage VD corresponding to a frequency shift FD. If there is a difference between the carrier frequency of the received FSK signal and the oscillation frequency of the local oscillation circuit 105, an output of the integrating circuit varies around the mean vale VD between when the data represents a mark and when the data represents a space as shown by waveform 155. Therefore, a control circuit 807 controls the voltage controlled oscillator 105 such that the output of the integrating circuit 805 approximately equal to the mean value VD to provide an automatic frequency control.
In the FSK signal transmission system, a high speed and multi-value transmission and decreasing a bandwidth per channel are required. That is, a modulation index of the FSK signal is required to be low.
If the FSK receiver as shown in FIG. 19 receives an FSK signal having a modulation index less than 1, the number of edges for one data interval of the I and Q base band signals becomes small. Then, there is a possibility that an edge may be disappear for one data interval of either of mark or space data if there is a difference between the carrier frequency of the FSK signal and the local frequency signal of the voltage controlled oscillator 105. Therefore, an error in the means value circuit 806 may occur, so that the automatic frequency controlling operation may be incorrectly.
FIG. 21 is a spectrum diagram of the prior art showing a spectrum of a four-value FSK signal. If the FSK receiver shown in FIG. 19 receives an FSK signal having four values, the AFC cannot operate correctly because the output voltage of the integrator 805 varies between when the frequency shift frequency is .DELTA.F and when it is 3 .DELTA.F though the carrier frequency approximately equal to the frequency of voltage controlled oscillator 105.
Further, there are needs for a simple structure and a low power consumption if the FSK receiver is employed as a portable receiver such as a pager.
In such portable receiver, the receiver is intermittently operated. The whole circuits of the receiver is supplied with a supply power only when a synchronizing signal is transmitted for providing a synchronization with a base station and when the FSK signal is transmitted toward the group to which that receiver belongs but a clock circuit should be constantly supplied with the supply power.